The present invention relates to the electro-hydraulic servovalve art and, more particularly, to a rate limiter which limits the rate of change of the control signal applied to an aircraft electro-hydraulic servovalve to limit peak hydraulic pressure to a specified maximum value.
Electro-hydraulic servovalve systems are found in many applications, including the aircraft control art. There, aircraft control surfaces, such as the elevators and rudder(s) are driven into position by hydraulic actuators, each of which is under the control of an electrical command signal. This command signal is produced as a result of the pilot's or the autopilot's inputs representative of a desired change in the aircraft's flight path.
A problem that is common to such electro-hydraulic servo control systems, especially in aircraft applications, is that the hydraulic peak pressures under transient conditions, both on the pressure and return sides, may exceed acceptable limits. In addition, the magnitudes of the transient pressure variations may significantly reduce fatigue life of the hydraulic tubing and other components. Since the hydraulic system, including the various hydraulic lines must be capable of handling such transient pressure peaks and variations without failure, it is desirable to minimize the transient pressures to thereby reduce the peak pressure handling and fatigue life requirements on the hydraulic system.
One approach to reducing hydraulic transient peaks is through the use of an accumulator. An accumulator is a mechanical tank which connects to the pressure (and/or return) side of the hydraulic system. A reservoir of hydraulic fluid within the tank acts on a piston-like device having hydraulic fluid on one side and a compressed gas on the other. As a transient pressure peak occurs in the line, the piston displaces against the pressure of the compressed gas, thereby limiting the maximum value of the transient peak hydraulic pressure.
While hydraulic system accumulators do act to reduce transient pressure peaks, they add significant weight, cost and maintenance requirements to an aircraft.
Another possible approach is to reduce the pressure transients is to increase the hydraulic line diameters. However, this approach imposes even higher weight penalty than the use of accumulators.
It should be noted that there are two other ways of reducing or controlling the maximum servovalve rates without using the servovalve current rate limiter, but these methods also have serious deficiencies.
In the first alternate method, system gain can be reduced to slow down the servovalve and thus indirectly reduce the maximum servovalve rate. However, this also results in unacceptable degradation of the actuator dynamic response characteristics.
In the second alternate method, a servovalve with the servo controlled system can be mechanically modified to have a reduced rate capability. However, the desired rate capability can be achieved only at a specific hydraulic system supply pressure and temperature. In practice, the hydraulic system supply pressure and temperature can vary significantly over various flight conditions. The mechanically achieved servovalve rate limiting capability will then often be either too high to adequately control the pressure peaks or too low to achieve the desired actuator dynamic response characteristics.
There is a need in the art, therefore, for an apparatus and method which inherently reduces pressure transients created in hydraulic systems, such as are found on aircraft, which does not rely on the use of an accumulator or other additional hydraulic device and which does not result in unacceptable degradation of the actuator dynamic response characteristics.